FIG. 6 is a sectional view illustrating a conventional three-way nomally closed type duty-driven solenoid valve as disclosed, for example, in Japanese Unexamined Utility Model Publication No. H7-34,271.
This conventional solenoid valve 100 comprises a main body 101 having a coil 102 wound on the outer periphery thereof and a guide hole 103 provided in the interior thereof, a plunger 105 made of a magnetic material which is slidably inserted into a sleeve 104 engaged with the guide hole 103 of the main body 101, a fixed iron core 106 made of a magnetic material, provided coaxially opposite to this plunger 105, a spring 107 which is provided in contact with the plunger 105 and imparts a force to the plunger 105 in a direction of separating the plunger 105 from the fixed iron core 106, and a rod 108 operably connected integrally to a side opposite to the fixed iron core 106 of the plunger 105.
The main body 101 is provided integrally with a valve body 101A. The valve body 101A comprises a ball 109 serving as the valve, an input side and a discharge side valve seats 110a and 110b with and from which the ball 109 comes into contact and is separated, an input port 111 for inputting a fluid pressure into the input side valve seat 110a, and an output port 112 for putting out the fluid pressure to outside. A throughhole is pierced in the fixed iron core 106 at the axial center position, to form a discharge port 113. A shim 114 made of a nonmagnetic material is provided between the plunger 105 and the fixed iron core 106.
In this solenoid valve 100, in a state in which current is fed to the coil 102, the force imparted by the spring 107 acts to press the plunger 105 in a direction toward the discharge side valve seat 110b. The plunger 105, guided by the sleeve 104, slides in the direction toward the discharge side valve seat 110b. Along with displacement of the plunger 105, the rod 108 moves toward the input side valve seat 110a, to bring the ball 109 into contact with the input side valve seat 110a. The input port 111 is thus closed to achieve communication between the output port 112 and the discharge port 113.
When current is fed to the coil 102, a magnetic attracting force acts to attract the plunger 105 toward the fixed iron core 106 against the force imparted by the spring 107. The plunger 105, guided by the sleeve 104, slides toward the fixed iron core 106. Along with displacement of the plunger 105, the rod 108 moves in a direction leaving the input side valve seat 110a. The fluid pressure acts from the input port 111 to the ball 109 which leaves the input side valve seat 110a and comes into contact with the discharge side valve seat 1110b. The discharge port 113 is thus closed, thus achieving communication between the input port 111 and the output port 112.
By thus controlling the power to the coil 102, opening/closing operation of the channel is accomplished. Pressure of the output port 112 varies as shown in FIG. 9 by changing the ratio of power fed to the coil 102, i.e., driving ratio (duty %). It is thus possible to control the pressure of the output port 112 to a prescribed pressure by controlling the driving ratio.
When the plunger 105 is magnetically attracted toward the fixed iron core 106 through feeding of power to the coil 102, the plunger 105 comes into contact with the fixed iron core 106 via the shim 114. At this point, there is a gap corresponding to the shim 114 thickness between the plunger 105 and the fixed iron core 106. When feeding of current to the coil 102 is discontinued, therefore, the effect of a residual magnetic flux retaining the plunger on the fixed iron core 106 side is reduced, and the plunger 105 is caused to rapidly move in a direction leaving the fixed iron core 106 by the force imparted by the spring 107.
In the absence of the shim 114, power feeding to the coil 102 brings the plunger 105 into contact with the fixed iron core 106. When power to the coil 102 is shut off, the residual magnetic flux serves to retain the plunger 105 on the fixed iron core 106 side. It is therefore necessary to set a large force imparted by the spring 107 so as not to be affected by the residual magnetic flux, and adjustment of this imparted force has so far been difficult.
In this solenoid valve 100, the pressure of the control fluid acting on the ball 109 serves to push up the plunger 105 onto the fixed iron core 106 side. If an imparted force capable of resisting to this pressure is not previously imparted to the spring 107, the force pressing down the ball 109 against the valve seat 110a could not resist to the pressure of the control fluid, so that the ball 109 leaves the valve seat 110a and the control fluid would flow in through the input port 111.
When assuming that the force of the control fluid pushing up the plunger 105 is P and the force imparted by the spring 107 is F:
When power is not fed to the coil (during OFF): F&gt;P PA1 When power is fed to the coil (during ON):
electromagnetic force+P&gt;F
It is therefore necessary to set electromagnetic force (magnetic attracting force) of the solenoid valve to a value&gt;F+P&gt;0.
In other words, the electromagnetic force must be larger than the force F imparted by the spring 107. When the pressure P of the control fluid is increased, the force F imparted by the spring 107 must also be increased. As a result, the size of the solenoid valve would be larger.
In the solenoid valve 100, furthermore, when the plunger 105 is attracted by an electromagnetic force toward the fixed iron core 106 side, the ball 109 moves apart from the valve seat 110a, the control fluid flows in through the input port 111. At this point, the force P acting on the plunger 105 via the ball 109 decreases, leading to a longer F-P value.
When controlling the control fluid of a large pressure, therefore, the ball 109 leaving the valve seat 110a results in an excessively large F-P value so that the plunger 105 cannot be attracted by an electromagnetic force. The plunger 105 is pushed back, and the ball 109 is moved in a direction of bringing the ball 109 into contact with the valve seat 110a. When the gap between the ball 109 and the valve seat 110a becomes smaller, the force P acting on the plunger 105 via the ball 109 increases, leading to a smaller F-P value, so that the plunger 105 is attracted by an electromagnetic force toward the fixed iron core 106. When controlling the control fluid of such a high pressure, an oscillation phenomenon comprising repeated up-down movement is produced in the plunger 105, thus causing a large sound.
When controlling the control fluid of a high pressure, therefore, it is necessary to increase the magnetic attracting force to prevent oscillation phenomenon of the plunger, thus leading to a larger solenoid valve.
The energizing current with which the plunger 105 begins to move by an electromagnetic force depends upon F-P. When the force F imparted by the spring becomes larger, the plunger 105 becomes harder to move, and the duty range in which the output port pressure becomes null in FIG. 9 becomes larger. When the force imparted by the spring becomes smaller, on the other hand, the plunger 105 becomes easier to move. The duty range in which the output pressure becomes null in FIG. 9 becomes shorter, and in the non-excited state, the plunger 105 moves along with oscillation of the solenoid valve, thus achieving communication between the input port 111 and the output port 112. In the solenoid valve 100, as described above, change in the force F imparted by the spring affects the properties. It has therefore been necessary to adjust the energizing current.
Further in this solenoid valve 100, upon feeding a current to the coil 102, the plunger 105 hits the fixed iron core 106 via the shim 114 under the effect of magnetic attracting force of the coil 102. Upon discontinuing power supply to the coil 102, the plunger 105 is pushed back by the force imparted by the spring 107, and the ball 109 collides with the valve seat 110a. A problem is therefore that two collisions occur in a period in feeding current to the coil 102, resulting in a big noise.
Because the shim 114 is arranged in a free state between the plunger 105 and the fixed iron core 106, the switching operation of the power to the coil 102 produces a play of the shim 114, and the shim 114 held between the plunger 105 and the fixed iron core 106 repeatedly suffers impact. A long-term use therefore causes wear of the shim, leading to breakage thereof.
Inflow of the control fluid into the space between the plunger 105 and the shim 114 and outflow of the control fluid through the space between the plunger 105 and the shim 114 occur repeatedly. When viscosity of the control fluid becomes higher at low temperatures, therefore, inflow and outflow resistance of the control fluid becomes larger, and this poses a problem of a difference in operating time from the operation at high temperatures and resultant change in properties at low temperatures. Further, depending upon the size of the shim 114, there may be posed another problem of adherence of the shim 114 to the plunger 105 in contact.
To avoid adherence described above, an improvement measure of achieving a line contact between the plunger and the shim is proposed, for example, as disclosed in Japanese Utility Model Publication No. H7-38,779, by making some contrivances for the end face shape of the plunger.
FIG. 7 is a perspective view illustrating a plunger in a conventional solenoid valve as disclosed in Japanese Utility Model Publication No. H7-38,779.
The plunger 105 has radial grooves 105a formed on an end face thereof. These radial grooves 105a are formed around a center hole 105b housing a spring 107 in the radial direction toward outside, adjacent to each other in the circumferential direction to cover the entire circumference.
In this solenoid valve, the plunger 105 and the shim 114 are in line contact which has a very small contact area. Even when viscosity of the control fluid increases, therefore, the control fluid is non-existent at the contact portion thereof, and the plunger 105 is not affected by the control fluid.
A long use however causes wear of the end faces of the plunger 105, change from line contact into plane contact between the plunger 105 and the shim 114, so that the plunger 105 is affected by the control fluid.
To avoid occurrence of cracks in the shim, an improvement measure of eliminating play of the shim by securing the shim to the plunger is proposed, as disclosed in Japanese Unexamined Utility Model Publication No. H4-74,780.
Even when the shim is secured to the plunger, however, the shim is subjected to repeated impact from the fixed iron core, and for some flaws or thickness of the shim, breakage of the shim is induced by a long-term use, and no perfect counter-measure has been available.